Born in Sydney, Australia, Farrell has been in and around racing cars since he was 11 years old, when he first started helping out his elder brother, who was racing Minis. As soon as he left school Farrell set off to Europe and spent a year working as a mechanic in Formula 3 racing before returning to Australia to attend the University of New South Wales to study aeronautical engineering. Throughout the course he prepared and raced cars and after graduation in 1983 he and his brother headed to Britain once again to race in Formula Ford 2000. Money was always the problem and in 1985 the Farrell Brothers established Milldent Motorsport in Hinckley, Leicestershire, to prepare cars for up-and-coming racers. Business boomed for a while and the business expanded to running Formula Ford 1600 and Formula Ford 2000 machinery, while the team also took on an Australia visitor called Malcolm Oastler.Oastler was soon snapped up by Reynard Racing Cars to work as the development engineer on the company's FF1600 program while Milldent was eventually sold and Farrell joined Chamberlain Engineering's sportscar team, based in Buntingford in Hertfordshire, running Spice chassis in the C2 class. Success in the class resulted in Farrell being recruited by Tom Walkinshaw's Silk Cut Jaguar team and in 1991 he engineered the World Championship-winning Jaguar of Teo Fabi.Sportscar racing, however, was fading by then and in 1993 Farrell moved to David Richards's Prodrive as chief engineer of the team's British Touring Car Championship team. This led to a job with Ray Mallock's factory Opel team before he returned to TWR to be chief engineer of the Nissan Le Mans project.In August 1998 Farrell was approached by his fellow countryman and former employee Malcolm Oastler and asked if he would like to join British American Racing. He took the opportunity and began working with the Tyrrell team, engineering Ricardo Rosset for the final races of the 1998 championship. In 1999 he was appointed chief engineer. He remained in that role until 2002 when he became head of mechanical design but later in the year he was transferred by the new team boss David Richards to be the technical director of the 555 Subaru World Rally Team.